A fuel cell converts chemical energy directly to electrical energy by supplying a fuel and an oxidant to two electrically-connected electrodes and causing electrochemical oxidation of the fuel. Unlike thermal power generation, fuel cells are not limited by Carnot cycle, so that they can show high energy conversion efficiency. In general, a fuel cell is formed by stacking a plurality of single fuel cells each of which has a membrane electrode assembly as a fundamental structure, in which an electrolyte membrane is sandwiched between a pair of electrodes. Especially, a solid polymer electrolyte fuel cell which uses a solid polymer electrolyte membrane as the electrolyte membrane is attracting attention as a portable and mobile power source because it has such advantages that it can be downsized easily, operate at low temperature, etc.
In a solid polymer electrolyte fuel cell, the reaction represented by the following formula (I) proceeds at an anode (fuel electrode) in the case of using hydrogen as fuel:H2→2H++2e−  Formula (I)
Electrons generated by the reaction represented by the formula (I) pass through an external circuit, work by an external load, and then reach a cathode (oxidant electrode). Protons generated by the reaction represented by the formula (I) are, in the state of being hydrated and by electro-osmosis, transferred from the anode side to the cathode side through the solid polymer electrolyte membrane.
In the case of using oxygen as an oxidant, the reaction represented by the following formula (II) proceeds at the cathode:2H++(½)O2+2e−→H2O  Formula (II)
Water produced at the cathode passes mainly through a gas diffusion layer and is discharged to the outside. Accordingly, fuel cells are clean power source that produces no emissions except water.
In the fuel cell, a decrease in voltage attributed to overvoltage is one of major causes of decreasing output. Examples of the overvoltage include activation overvoltage derived from an electrode reaction, resistance overvoltage derived from the resistance on an electrode surface or the resistance of the fuel cell, and concentration overvoltage derived from concentration distribution of the reactant on the electrode surface. The electrocatalyst exerts the effect of decreasing activation overvoltage among the above-mentioned overvoltages.
Platinum and a platinum alloy are preferably used as the electrocatalyst in the cathode and anode of the fuel cell because the platinum has high catalytic performance. However, slow reaction rate of oxygen reduction in the cathode using the conventional platinum catalyst and high platinum cost cause a significant barrier to the commercialization of fuel cells. As the catalyst for solving such a problem, a particle composite containing palladium or a palladium alloy covered with an atomic thin layer of a platinum atom is disclosed in Patent Literature 1.